340 



Optical and Bloicpipe Examination of Chlorite. 



optic axes in the angle of the plate opposite to the base, and fonnd 

 them to have an equal inclination with b\ and the plane of these 

 axes was found to form an angle of about 60^^ with the plane of 

 the others, or to be at ri2:ht angles with one of the sides of the 

 triangle, (which is as near the angle as could be determined by 

 marking the direction upon the plate and subsequent measure- 

 Tnent by goniometer and protractor,) this peculiar relation o{ two 

 systems of optic axes, had been noticed in a also, arid there is 

 probably a line of composition in .most of the crystals from the 

 locality. The position of this line is represented by the shorter 

 dotted line in the figure. 



Another interesting peculiarity is, that the optic axes are not 

 equalhj inclined to the cleavage surface^ or to a Hue perpendicular 

 to it, (the '^normal" of Biot.) The inclinations were measured, 

 but as the instrument had not been adapted to this mode of meas- 

 urement, the angles given can be regarded onl}^ as approxima- 

 tions, and are here given merely to show the existing inequality 

 of the inclinations- 



Spec, a gave the angles 



(( 



b 



a 



(C 



it 



50^ and 34^. 



58^-13' and 27^-40^ 



The greater angle being on the side of the ^^ normal" adjoining 



the base of the plate or triangle. 



From these results the mineral must be referred to one of the 

 systems of crystallization having the three axes unequal, — and it 

 cannot therefore be classed with the species chlorite or ripidolite, 

 which according to authors is rhombohedral or hexagonal. The 

 Ala chl(»rite was examined optically by Biot and reported to be 



uviaxiaL 



It is here interesting to observe that we have this undoubtedly 

 clinometric mineral with such a peculiarly high angle between 

 the optic axes, occurring in triangular plates and masses so much 

 resembling the micas from Monroe, N. Y.; whose biaxial charac- 

 ter is so difficult of determination, and which by reason of this 

 form have been referred by some eminent crystallographers to the 

 rhombohedral system. The form in both cases may be consid- 

 ered as resulting from an acute oblique rhombic prism by the 

 replacement of the acute solid angles.* 



Examined with the blowpipe the mineral gives the following 



reactions. 



B.B, in the forceps, contracts and becomes opaque and white 



? 



with traces of fusion on the edses. 



as with forceps. In an open 

 white rin^ is formed near the 



tube, 



Alone, on charcoal, same 

 gives off water, and a 

 when strongly heated. 



-... rmg is lormea near me assay , ^, 



With borax in the oxydating flame, dissolves readily with m^^^^ 

 ebullition; the glass while hot, red and brownish, but becomes 



X 



Dana's Mineralogy, 1st edition, p. 264r, and Am. Jour Scl, 2d Ser., xii, p- 



8. 



I 



- ^ 





